Move Block table integration to a submodule

This is a bit of an odd request I realize, so bear with me for a second:

Motivation

When used with a large number of beans (say creating those beans with use on an entityreference field), the _block_rehash function becomes a huge performance problem in an unpatched block module. This issue references the problem with 5000 blocks, and patches the block table query in order to make the query take much less time.

There are still some related issues around that, as bean_load_all_beans() can take a significant amount of time, and in certain entityreference situations (like using the Paragraphs module to provide the reference), _block_rehash could be called several times in a given editorial page load (28 times observed in our case). Every 3 of those the static cache would be cleared and bean_load_all_beans() would occur again.

In order to increase performance, we realized that we were not actually using beans as ...well.... blocks. We were only using beans as a generic entity which already had some pretty good integrations into other things (IEF, Panelizer, etc), and not placing them as blocks on various block-system controlled pages.

"Solution"

So, we decided to break the block table integration into a sub-module, and enable that module by default in an update hook (though I don't believe I added anything to turn it on by default on bean module install). This moves I think everything related to the core block table into bean_block, and adds an additional hook to facilitate the cache clearing if the module is enabled.

Footnote

I don't actually expect that this patch is going to be approved, but I figured it's useful to put it up here for review in case someone else has a similar use-case.